This invention relates to electroluminescent devices containing polymers, and to processes for the use of these devices.
In recent years, a great deal of research has been conducted into electroluminescent materials, that is to say materials which emit electromagnetic radiation (typically visible light) when an electric current flows through the material. Electroluminescent materials are potentially useful for the construction of image display devices, which could be very thin and lightweight, and could thus advantageously replace cathode ray tubes, gas plasma displays, liquid crystal displays and other types of image display devices.
Several different types of electroluminescent materials are known; see, generally as to development of such materials, International Patent Application No. PCT/GB90/00584 (Publication No. WO 90/13148). The first type to be developed was inorganic semiconductor materials such as gallium phosphide and zinc sulfide. However, such inorganic electroluminescent materials are not readily usable in large image display devices, and many of them suffer from practical drawbacks, including poor reliability. Accordingly, much recent research has concentrated on organic electroluminescent materials.
Many organic compounds, especially polycyclic arenes such as anthracene, perylene, pyrene and coronene, are electroluminescent. However, electroluminescent devices using these monomeric organic compounds suffer from poor reliability, and these organic compounds present difficulties in preparing the thin layers of the materials needed for use in practical electroluminescent image display devices, and the electrodes needed for electrical contact with such layers. Techniques such as sublimation of the organic material produce layers which are soft, prone to recrystallization and unable to support high temperature deposition of electrode layers, while techniques such as Langmuir-Blodgett film deposition produce films of poor quality, dilution of the active material and high cost of fabrication. Prior art electroluminescent devices formed from these materials, such as that described in U.S. Pat. No. 3,621,321, typically suffer from high power consumption and low light output.
Attempts have also been made to use solid solutions of non-polymeric organic electroluminescent materials in non-electroluminescent polymers as the active layer in electroluminescent devices; see, for example, U.S. Pat. No. 4,356,429. However, use of such solid solutions carries a substantial risk of phase separation by crystallization of the electroluminescent material out of the polymer, especially in environments where the electroluminescent device may be subjected to large changes in temperature. In addition, often it is difficult to find a non-electroluminescent polymer which can both conduct electricity and dissolve a large proportion of the active electroluminescent material to form the necessary solid solution. Finally, the use of a solid solution necessarily involves substantial dilution of the active electroluminescent material and thus lowers the maximum light flux from a given area of the electroluminescent device.
Accordingly, research has been carried out on electroluminescent polymers having an electroluminescent group incorporated into the polymer itself. These polymers do not suffer from the phase separation, solubility and stability problems encountered with the aforementioned solid solutions. Among the polymers which have been tested for use in electroluminescent devices are poly(3-substituted thiophenes); see, for example:
Braun et al., Electroluminescence and electrical transport in poly(3-octylthiophene) diodes, J. Appl. Phys. 72(2), 564 (1992);
Ohmori et al., Visible-Light Electroluminescent Diodes Utilizing Poly(3-alkylthiophene), Jap. J. Appl. Phys. 30(11B), 1938 (1991) (hereinafter "Ohmori I"); and
Ohmori et al., Effects of alkyl chain length and carrier confinement layer on characteristics of poly(3-alkylthiophene) electroluminescent diodes, Solid State Commun. 80(8), 605 (1991) (hereinafter "Ohmori II").
However, the electroluminescent performance of the poly(3-alkylthiophenes) described in the above papers is inferior to certain other electroluminescent polymers known in the art.
It has now been found that the electroluminescent performance of poly(3-alkylthiophenes) is greatly affected by the head-to-tail ratio of the polymer, and that improved electroluminescent performance can be achieved using a poly(3-alkylthiophene) having a head-to-tail ratio of at least about 80 percent.